This invention relates to a bending wave panel speaker and a method of driving such a speaker.
WO97/09842, and U.S. counterpart application Ser. No. 08/707,012, filed Sep. 3, 1996 (now U.S. Pat. No. 6,332,029), which is incorporated herein by reference in its entirety and assigned to the present applicant, New Transducers Ltd., discloses bending wave loudspeakers; such a loudspeaker is illustrated in FIG. 1. In general, a bending wave loudspeaker includes a panel 1 with at least one exciter 3 coupled to the panel 1 at one or more discrete points or small regions. The exciter position or positions is or are selected to drive distributed resonant bending wave modes to cause the panel to emit sound.
Prior art arrangements with discrete exciters have a disadvantage in that it may be difficult in some applications to locate the exciter at the desired preferential locations, as taught in WO97/09842, and counterpart U.S. application Ser. No. 08/707,012. For example, the loudspeaker may be required to be installed in existing equipment, and the required transducer location may not be possible if another component gets in the way. Alternatively, in a transparent speaker using a transparent panel, it may be difficult to position an exciter in a preferred location without creating visual intrusion, since it is difficult to make conventional exciters transparent.
Thus, it would be advantageous to gain the benefits of preferential exciter placement without in fact needing to mount exciters at the preferred locations.
In the related PCT application No. WO00/02417, likewise to the present applicant but published after the priority date of the present application, several arrangements using a transparent loudspeaker panel and an exciter coupled to the panel are disclosed. The exciter may comprise a piezoelectric or electric material over the panel.
Another prior application which describes a transparent flat panel speaker is GB 2052919 to Hitachi Ltd. This application describes a transparent piezoelectric speaker with a piezoelectric layer on one face. As explained in GB2052919, a problem in such arrangements is that the loudspeaker only operates over a narrow frequency band. Although in GB 2052919 some improvement is obtained by choosing an elliptical shape of loudspeaker panel, the results are still less than optimalxe2x80x94the best results presented have little sound output outside 1 kHz to 3 kHz, a very narrow band.
GB 2052919 teaches that essentially only one mode is excited, unlike the arrangements of W097/09842 in which a number of modes at different frequencies may be excited. However, for a good acoustic output over a range of frequencies using resonant bending wave modes, the exciter should excite a number of resonant modes that are distributed in frequency.
Thus, there is a need for improved performance if such speakers are to be useful for any but the most basic of applications.
According to a first aspect of the invention there is provided a bending wave loudspeaker comprising a panel capable of supporting bending waves and having opposed faces, a transducer extending over a large fraction of one face of the panel and coupled to the panel surface, and a constraint coupled to a discrete small region of the panel constraining the movement of the panel, so that activating the transducer material can excite a plurality of resonant bending wave modes of the panel.
Instead of placing the transducer at a predetermined location, the transducer is spread over a significant part of the panel area and the panel constrained at one or more constraint locations. The large fraction of the panel surface may be at least 60%, preferably 75% or even 90% of the area of the panel. The larger the fraction, the larger the transducer and hence the larger the output power possible. This is particularly useful when the transducer material only provides a small motion for a unit input, as is often the case for piezoelectric material.
The large fraction is preferably substantially the whole surface of the panel and each small region may be small in comparison with the area of the panel.
Each small region may be no more than 10% of the area of the panel, preferably no more than 1%. Furthermore each small region may have a linear size no greater than 20% of the width of the panel, preferably no greater than 10% and further preferably no greater than 4%. Too large a constraint may result in a panel that is very hard to bend and so which is very hard to drive.
The panel may be a panel of a material that is particularly suitable for supporting resonant bending waves in a predetermined operative frequency range.
As mentioned above, one problem that gave rise to the less than adequate results in GB 2052919 is that the piezoelectric exciter did not excite a good distribution of resonant bending wave modes.
In the loudspeaker according to a preferred aspect of the invention, the provision of a localised constraint allows reasonable or good excitation of a plurality of resonant bending wave modes by a transducer that is extended over the surface of a panel.
The transducer may comprise a sheet of transducer material extending over a large fraction of one face of the panel and coupled to the panel surface.
In another aspect of the present invention, a second transducer may be applied to the opposite face of the panel, in what is known as axe2x80x9cbimorphxe2x80x9d configuration. The further transducer may comprise a sheet of transducer material extending over the large fraction of the opposite face of the panel to the first transducer.
A bimorph configuration provides a number of additional advantages. Firstly, the plate is then sandwiched between two transducer sheets; these can be arranged so that the top sheet shrinks as the bottom expands to provide a true bending stress to the plate, rather than just a linear stress applied to one face as occurs in arrangements with only one sheet of transducer material (known as a unimorph).
Moreover, the panel and transducer sheets then form an integral unit which can be optimised as a unit, for a good distribution of resonant bending wave modes.
The constraint may be a mass fixed to the panel, for example on one face or embedded within the panel.
The constraint may also be a rigid coupling piece coupled to the panel over a small region of the panel for substantially impeding movement of that small region.
The constraint locations may be selected so that the resonant bending wave modes of the constrained panel, especially those at the lower end of the operative frequency range, are spaced beneficially for a desired acoustic result. The location and parameters of the constraint may be selected to substantially optimise the acoustic output.
The locations of the constraints may be determined by mathematical or numerical methods, or even systematic experiment.
In a preferred aspect of the invention, the constraints can be located at suitable locations for mounting a conventional small exciter on a free panel. Rather than drive a free panel at a discrete location the panel is driven over a large fraction of its surface and xe2x80x9cpinnedxe2x80x9d at the location that would be suitable for driving it using a local transducer. Thus, the loudspeaker according to this embodiment of the invention is effectively an inverse of a conventional distributed mode loudspeaker, with a localised constraint instead of a localised transducer.
In yet another embodiment, the constraints may be located away from the edges, i.e. at least 20% of the width of the panel away from the edges. By width, it is meant the distance across the panel, in a direction generally orthogonal to the length of the panel. Alternatively, the constraints can be located at asymmetric locations. If the panel is of symmetric form having one or more axes of symmetry the constraints may be spaced away from one or all of the axes of symmetry, for example by a distance of at least 3% of the width of the panel, preferably at least 5%.
Such a constraint location is not always possible. Accordingly, the constraints may be located at the edges of the panel, or at least located no further from the edge than 20% of the distance across the panel from the edge. This is particularly useful in the case that the central region of the panel is required to be transparent.
The sheet or sheets of transducer material may be sandwiched between a pair of electrodes to make a transducer film. The electrodes may be deposited on either side of a transducer film; and they may be transparent. One suitable transparent electrode material is indium tin oxide.
A transducer film can be glued to cover one or both faces of the panel. One electrode of the pair of electrodes may cover the large fraction of one face of the panel and mechanically couple the transducer material to the panel.
The transducer material can be shape-changing when electricity is applied. Accordingly, the material can be a piezo-electric material, such as lead lanthanum zircouite titanate (PLZT) or polyvinylidene fluoride PVDF.
In a preferred embodiment, the panel and the piezoelectric material can be transparent. This is particularly useful combined with transparent panels.
The panel can be suspended or otherwise supported in such a way that the supports have as little effect on the resonant modes as possible.
Alternatively, the panel can be supported by the constraints, or the panel can be supported on a frame as is conventional in bending wave panels. In the latter case, the constraints may simply restrain panel movement at predetermined locations.
In another aspect, a method of making a bending wave loudspeaker having a panel with opposed faces, includes determining the shape, size and properties of a panel, selecting the properties of a sheet of transducer material to be applied over a selected large fraction of a face of the panel, selecting the location of at least one small region and the parameters of the at least one constraint to be applied to the panel on the at least one small region so that the panel provides useful acoustic action, and making a loudspeaker from a panel as determined by applying the selected transducer material over the large fraction of a face of the panel, and applying selected panel constraints at the selected small region using the selected constraint parameters.